1. Field of the Invention
This invention relates to a method of smoothing image signals. This invention particularly relates to a method of smoothing image signals by use of a median value smoothing technique.
2. Description of the Prior Art
As a method of eliminating noise contained in an image, various image signal smoothing methods based on spatial filtering processing have heretofore been known. Among the image signal smoothing ordinarily methods, mean value smoothing is used. In the mean value smoothing, the image signal at every picture element representing a two-dimensional image is replaced by a mean value of the image signal at said picture element and the image signals at a plurality of the picture elements in the vicinity of said picture element. Though mean value smoothing has the advantage that calculation processing is very easy, it is disadvantageous in that blur of the edge component readily arises.
As a smoothing method suitable for eliminating the aforesaid problem, median value smoothing is known. In median value smoothing, the image signal at every picture element representing a two-dimensional image is replaced by a median value of the image signal at said picture element and the image signals at a plurality of the picture elements in the vicinity of said picture element. For example, as shown in FIG. 3, image signals at picture elements Pf, Pg, Ph and Pi adjacent to a picture element Pe are designated by f, g, h and i, the maximum value among the image signals f to i is f', followed by g', h' and i', i.e. f'.gtoreq.g'.gtoreq.h'.gtoreq.i'. In this case, when an image signal e at the picture element Pe satisfies the condition e&gt;g', the image signal e is replaced by the median value g'. When e&lt;h', the image signal e is replaced by the median value h'. When g'.gtoreq.e.gtoreq.h', since the median value is equal to e, the original image signal e is employed directly. As the image signals at the picture elements in the vicinity of a picture element which are used for calculating the median value, besides the image signals at the four neighboring picture elements, it is possible to employ, for example image signals at two picture elements adjacent to each other in one direction, image signals at eight neighboring picture elements, or image signals at 24 neighboring picture elements
However, in the aforesaid conventional median value smoothing, the image signals after the median value replacement change unnaturally as compared with the original image signals, and therefore the edge component is distorted. The problem will hereinbelow be described in detail with reference to FIGS. 4 and 5. In the most simple case, smoothing is conducted by use of the image signal which is to be smoothed, and the image signals at two adjacent picture elements. In the case where original image signals a, b, c and d at picture elements Pa, Pb, Pc and Pd standing in one direction as shown in FIG. 4 attain values as shown in FIG. 5, since the relationship among the adjacent image signals a, b and c is a&lt;c&lt;b, the original image signal b is replaced by c=b0. Also since the relationship among the adjacent image signals b, c and d is c&lt;b&lt;d, the original image signal c is replaced by b=c0. In this case, the original image signals b and c having the relationship of b&gt;c are changed by median value replacement so that c0&gt;b0. Thus the relationship between the original image signal values is reversed by median value replacement. Therefore, when the image is reproduced by use of the image signals a, b0, c0 and d, distortion of the edge component arises as mentioned above.